1. Field of the Invention
The present invention relates to digital modems and more particularly to the detection of predetermined reference word sequences during the initial training procedure between two modems, or during intermediate phases of retraining or modification of the rate.
2. Discussion of the Related Art
In digital modems, the data to be transmitted are digitally encoded from a determined number of symbols and then transmitted as portions of sine wave signals whose phase and amplitude are modulated. Each symbol, for example 128 in accordance with the V32bis standard, corresponds to a signal having a predetermined phase and amplitude. To illustrate this conversion, the symbols can be disposed in a constellation such as illustrated in FIG. 1 where each symbol is disposed at a point corresponding to the phase and amplitude of the modulated signal representing the symbol. Thus, as represented in FIG. 2, each symbol is transmitted as a sine wave portion at a carrier frequency of, for example, 1800 Hz, during bit intervals corresponding to a baud rate of, for example, 2400 Hz. The sine wave is formed and decoded from a sampling frequency that is higher than the baud rate and is a multiple of the latter, for example 9600 Hz.
After decoding, each received Symbol is transformed into a digital word M including a first portion corresponding to the real value of the transmitted word and a second portion b corresponding to the imaginary value of this word. Thus, each word can be expressed by: EQU M=a+jb.
Due to the very high exchange rates between two modems, the quality of the telephone connection becomes increasingly important in order to provide reliable transfers of data between two modems. Possible impairments, such as attenuation of the high frequencies of the spectrum, near and far end echo with frequency offset, phase jitter and noise, influence the maximum bit rate possible for minimum errors. In the above V32bis standard, the demodulated complex signal received by a modem is coded on amplitude and phase and a constellation such as the one represented in FIG. 1 contains 128 possible values designed to be transmitted at a data rate of 14,400 bits/second at a baud rate of 2400 Hz. When the above mentioned impairments become more important, the difference between the received point and the ideal receive point, or receive error, becomes increasingly large until, eventually, the decision mechanism of the receiving modem confuses two or more adjacent points, thus causing intolerable errors in the reception.
In contrast, a connection with a slower data rate, such as 7,200 bits/second uses a constellation including only 16 values such as represented in FIG. 3 and permits a much larger tolerance to line impairments. These impairments may vary during the communication in which case the initial transmission rate has to be increased or decreased to find the optimal value. In general, a micro-controller is connected to the modem and can calculate the quality of the received signal, this value corresponding to the inverse of the average receive error. If this error becomes too important, the CCITT recommendations provide for a rate negotiation procedure which is quite short in duration to interrupt data transmission for a very short time of 288 bauds (120 ms) plus the round trip delay between the two modems.
The rate negotiation signal more particularly contains a preamble constituted by a sequence of predetermined signals, the sequence being repeated a determined number of times. Usually, this sequence corresponds to a succession of signals AA for the calling modem and to a succession of signals AC for the called modem. This sequence is repeated 56 times in the preamble. Considering the transmitted signals as a series of sine wave portions, a sequence of signals AA corresponds to an ideal sine wave at a 1800-Hz frequency, and a sequence of signals AC includes two components having a frequency of 600 and 3000 Hz, respectively.
A conventional process to detect the above successions of signals AA or AC uses two banks of digital filters at the front end of the receiver block of the modem. As represented in FIG. 4, the lower bank includes a prefilter F that allows only the frequencies ranging from 600 to 1800 Hz to pass, an energy calculator (providing the absolute square value of the signal) and a low-pass filter LPF1 of the first order. The upper bank includes a highly selective band-pass filter BPF centered on 600 or 1800 Hz followed by another energy calculator and by a low-pass filter LPF2. The outputs of the low-pass filters LPF1 and LPF2 are transmitted to a comparator 10. When sequences of signals AA or AC are present and when the band-pass filter BPF is centered on 600 or 1800 Hz, respectively, the energy in the upper bank is comparable to the energy in the lower bank. When a data signal is present, the upper bank provides a much lower level than the lower bank. This conventional process is satisfactory for the detection of sequences AA and AC during the initial handshake or retrain procedure where the duration of the analysis is relatively long. However, for operations to be carried out rapidly such as a rate negotiation operation, this procedure is less satisfactory since, in particular, it needs a relatively narrow band-pass filter BPF, which involves a relatively long response time. If it is desired to decrease this response time, selectivity is decreased, which results in a higher risk of false detection.